An uninterruptible power supply (UPS) provides battery backup in case mains power fails, such as due to a power outage. In general, a UPS plugs into a wall outlet providing alternating current (AC) mains power, and a system like a computing system such as a computing device plugs into the UPS. While mains power is operational, the UPS indirectly or directly powers the connected system via the mains power, while at the same time charging its batteries. When mains power fails, the connected system receives power from the UPS's batteries until the mains power returns or the batteries become depleted.